Many moisture-curable compositions cure in the presence of moisture to form crosslinked materials. The moisture may be obtained from water vapor in the air or from the substrate to which the moisture-curable composition is applied. Moisture-curable compositions may be used to provide adhesives, sealants, coatings and putties for numerous industries including aerospace and automotive industries. Moisture-curable compositions are advantageous because they do not require heat or solvents to cure.
Moisture-curable composition may comprise, for example, alkoxysilane or isocyanate functional polyurethanes, the alkoxysilane or isocyanate groups providing the active sites for the curing reaction with water. Alkoxysilane functional polyurethanes are preferred over isocyanate functional polyurethanes because they are less toxic, and generally provide better storage and faster cure properties. In addition, isocyanate functional polyurethanes may react with water to form carbon dioxide gas which can cause foaming and a decrease in the strength of the composition.
Numerous methods are known for preparing alkoxysilane functional polyurethane materials. One method, the condensation of an isocyanate functional material with a hydroxy functional alkoxysilane, proceeds as generically illustrated below: ##STR1## Advantageously, the resulting polyurethane is terminated at both ends with the moisture-reactive alkoxysilane groups to provide optimal curing and enhanced physical properties in the crosslinked material.
One problem with this method, however, is that the hydroxy functional alkoxysilane starting material can undergo an alcohol interchange reaction prior to reaction with the isocyanate functional polymer. For further discussion of alcohol interchange, see Metal Alkoxides, D. C. Bradley, R. C. Mehrotra, and D. P. Gaur, Academic Press, New York, 1978. See also Russian Journal Article (Zhurnal Obshchei Khimii Vol. 41, No. 4, page 933, April, 1977) which corroborates the alcohol interchange reaction for hydroxy functional alkoxysilanes having carbamate linkage. The article describes the reaction of ethylene carbonate and aminosilanes to form hydroxyethyl(silylalkyl)carbamates, stating that when ". . . there are alkoxy substituents on the silicon atom, the main reaction is accompanied by processes of inter- and intra-molecular polycondensation with formation of oligomeric products".
The alcohol interchange (i.e., intermolecular polycondensation) reaction is illustrated generically below: EQU 2HOSi(OR).fwdarw.HOSi-OSi(OR)+ROH
The products formed from the alcohol interchange reaction react with a diisocyanate as follows: ##STR2##
The resulting polyurethane bears an inert terminus (COOR) that does not participate in the crosslinking reaction with moisture. As a consequence, the resulting polyurethane may not sufficiently cure, thus remaining too tacky and providing inadequate strength for some applications.
A need therefore exists for hydroxy functional alkoxysilane materials which are contaminated with minimal amounts of nonlo functional alcohol (ROH). Such materials could be used to provide alkoxysilane functional polyurethanes which overcome the disadvantages associated with isocyanate functional polyurethanes and have good curing and physical properties.